This application pertains to a drive for a rotary agitator. In particular, the drive provides for agitation of chemicals in high-temperature, high-pressure environments without the need for a seal around the drive shaft, and allows for the use of conventional bearings to support the shaft.
Industrial chemical processes often occur in reactor vessels and require agitation to aid chemical reactions. For example, agitation may provide for homogenous mixing, or for uniform suspension of materials having different densities or phases such as emulsions or solids suspended in a liquid. In general, agitators typically include one or more propellers or impellers inside the vessel that are attached to a rotating shaft. The shaft extends out through the wall of the vessel to a motor that rotates the shaft and, in turn, rotates the impellers or propellers. One or more bearing assemblies, generally near the vessel wall, hold the shaft in place and allow it to rotate freely and steadily under various rotational, transverse, and thrust loads.
It is desirable that an agitator provide consistent performance with few failures. Major industrial processing plants are extremely complex and very expensive to operate. A breakdown at one vessel can stop the operation of a major portion of a plant, and disassembly (and reassembly) of an agitator drive for repairs often takes a long time and can destroy the batch being processed in the vessel. Even worse, a breakdown in the middle of a batch may require that the vessel be carefully and laboriously cleaned before processing may resume.
Where the conditions inside the vessel are severe, such as where the temperature and pressure inside the vessel are both very high, a conventional agitator drive system may not provide acceptable reliability. For example, the motor for a drive system is typically located in a low-pressure area, and the drive shaft passes from the motor into the vessel so that there generally must be seals, packing, and/or bearings at the point where the shaft passes through the wall of the vessel. Seals and packing are prone to quick degradation under severe conditions where they are placed in high temperatures or across high-pressure differentials. In addition, the seals, packing, or bearings must be properly lubricated, and under severe conditions, the lubricants may degrade or may even leak into the interior of the vessel, contaminating the process.
Conventional solutions may not be adequate to address such problems caused by severe conditions. For example, pusher mechanical seals are often used at the vessel wall between areas of high and low pressure. These seals generally rely, however, on elastomers, which are inappropriate materials for high-temperature applications. Metal bellows (or non-pusher) seals are often used where high temperatures are expected, but they do not generally work well under high pressures. Packing materials may also be provided around a shaft where it enters a vessel. While such a solution again works well under high pressure, it can cause problems where temperatures are elevated. For example, high clamping forces around the packing material help form a tight seal that can withstand high pressure, but the forces also create friction that produces additional heat. When combined with high temperatures in the vessel, this friction can cause rapid destruction of the materials.
Placing the drive systemxe2x80x94motor and allxe2x80x94entirely inside the vessel solves the problem of sealing across a high-pressure differential, but it is not generally acceptable. The drive motor will likely be less amenable to severe conditions than are the bearings that support the shaft because it contains bearings and other components that may not handle high temperatures or a corrosive environment well. And placing the entire drive system in the vessel simply places the bearings entirely inside the high-temperature, and potentially corrosive, conditions. In addition, access to the drive is more difficult when it is entirely inside the vessel. Moreover, the problem of potential contamination of the vessel may be worsened, particularly where the motor is hydraulically powered.
One solution to the problem is to break the shaft in two, placing the motor and part of the shaft outside the vessel, and the other part of the shaft inside the vessel, so that no portion of the drive passes through the vessel wall. The two parts of the shaft may be coupled through the vessel wall magnetically. The motor""s shaft outside the vessel may be attached to large magnets, and the drive shaft attached to the agitator inside the vessel may be attached to matching magnets. The sets of magnets may be positioned on each side of a protruding area of the vessel wall so that rotation of the motor induces rotation of the agitator by magnetic coupling.
This xe2x80x9cmagnetic couplingxe2x80x9d approach, however, is expensive and allows only limited torque to be delivered to the agitator, and still requires that the bearings supporting the shaft be located in the hostile environment of the vessel. As a result, it too may require that the bearings be made of special, expensive materials, may result in premature bearing failure, and may produce contamination of the vessel. Moreover, because the coupling force is inversely proportional to the square of the wall thickness between the magnets, there will be a practical limit to the level of coupling that can occur through a wall that is thick enough to maintain the integrity of the vessel. Furthermore, as torque requirements increase, the magnets may need to be placed further from the shafts so that the container through which the magnets operate must get larger, and its wall thickness must increase to contain the vessel pressure. As a result, practical torque and size limitations constrain the general applicability of magnetically coupled drives.
Accordingly, there is a need for an agitator drive system that can provide reliable operation to vessels that house severe conditions with little or no risk of pressure loss or of contaminating the contents of the vessel. In addition, there is a need to provide such a drive in a sealless system that can use conventional materials and parts. Furthermore, there is a need to provide a motor for such a drive that can operate reliably in a high-pressure atmosphere in which the pressure varies over time.
In general, an agitator drive is disclosed for use with a chemical processing vessel. The drive may be sealed with the vessel, and may thus be under the same or similar pressure as the housing. The drive may be separated from the housing by an insulated floor, so that the temperature inside the housing is significantly lower than that inside the vessel. A pair of overlapping shields, in the form of a standpipe attached to the drive floor and a skirt attached to the shaft, may prevent fluid from the housing from entering the vessel. In addition, the bearings that hold the shaft may be immersed in one or more sumps around the shaft that are filled with lubricant. As a result, the drive shaft does not pass from a high-pressure area to a low-pressure area, and the active drive components, such as the bearings and motor, are isolated from the high temperatures in the vessel.
In one embodiment, a drive for an agitator assembly has a drive housing that defines an interior volume and a drive motor mounted in the interior volume. A bearing support is connected to the inside of the housing and has a bearing receptacle in which one or more bearings are mounted. A drive shaft surrounded by a sump is rotatably mounted in the bearings and is driven by the drive motor. The sump defines an inner volume for holding a lubricant in which at least a portion of the bearings are located in the inner volume so that the bearings may be immersed in the lubricant. The sump may also be connected to the drive shaft and rotate with the drive shaft.
The bearing support may have an upper portion, which may comprise a plurality of support arms or a solid disc, outside the sump, and a cantilevered portion, which may comprise a solid cylinder, that contains the bearing receptacle. The shaft, one or more bearings, and cantilevered portion may define a fluid galley, and the bearing support may have a fluid supply manifold. A floor may be located below the sump and may have a passage surrounded by a standpipe through which the drive shaft passes. A skirt attached to the shaft may surround and overlap with the standpipe, and may be provided with a baffle that overlaps with a baffle on the standpipe. A fluid circulation system may also receive and recirculate fluid from the motor and the bearings.
In another embodiment, a drive is provided having a housing with a floor, and a drive motor in the housing connected to a drive shaft that extends through the floor. A standpipe around the drive shaft may extend upward from the floor, and a shield may be attached to the drive shaft above the standpipe to prevent fluid from entering the standpipe. The shield may comprise a skirt that overhangs the standpipe, and the skirt may have a baffle that overlaps a baffle on the standpipe. A drain port may be located in the housing near the floor. A sump may be mounted around the shaft, and may have a bearing mounted in its interior volume. The sump""s top edge may be above the top of the bearings, and the bearings may be cantilevered into the sump on a bearing support, which may have fluid passages for introducing lubricant near the bearings. The drive may also be provided with a media introduction line that extends through the standpipe and terminates near the shaft. Furthermore, the floor and a portion of the shaft that extends into the vessel below the floor may be insulated.
In yet another embodiment, a method of extending bearing life in an agitator drive is disclosed by which one or more bearings in a bearing support are provided around a drive shaft, a sump is provided around the bearings, lubricant is passed through the bearings until the sump is overflowing to lubricate and cool the bearings, and the fluid is recirculated. A divider, such as a floor, may be provided below the sump between the vessel and the drive around the drive shaft. Also, a standpipe may be provided around a hole in the divider to prevent fluid from passing into the vessel, and the fluid may be cooled outside the agitator drive.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.